Short arc high-pressure discharge lamp

ABSTRACT

A short arc high-pressure discharge lamp ( 1 ) for dc operation, includes a discharge vessel ( 2 ) that has two necks ( 4 ) diametrically opposite each other, in which an anode ( 26 ) and a cathode ( 7 ) made of tungsten are melted in a gastight manner, and which has a filling made of at least one noble gas and possibly mercury. According to the invention, at least the material of the cathode tip ( 11 ) contains lanthanum oxide La 2 O 3  and at least another oxide from the group consisting of hafnium oxide HfO 2  and zirconium oxide ZrO 2  in addition to the above-mentioned tungsten.

TECHNICAL FIELD

The invention relates to a short arc high-pressure discharge lamp fordirect current operation, having a discharge vessel which includes twodiametrically opposite necks, into which an anode and a cathode, in eachcase made from tungsten, are fused in a gastight manner and whichcontains a fill comprising at least one noble gas and optionallymercury. Lamps of this type are used as mercury arc lamps in particularfor microlithography in the semiconductor industry, to expose wafers,and as xenon arc lamps for cinema and video projection.

Prior Art

The mercury short arc high-pressure discharge lamps which are used forthe exposure process must supply a high light intensity in theultraviolet wavelength region—in some cases restricted to a fewnanometers of wavelength—with the light generation being restricted to asmall spatial area.

Intensive light generation within an extremely small space is likewise ademand imposed on xenon arc lamps for cinema and video projection.

The resulting demand for a high luminance can be achieved by a directcurrent gas discharge with a short electrode spacing. This produces aplasma with a high light emission in front of the cathode. The strongintroduction of electrical energy into the plasma generates electrodetemperatures which, in particular in the case of the cathode, causedamage to the material.

For this reason, cathodes of this type have hitherto preferablycontained a doping of thorium oxide ThO₂, which is reduced to thorium Thduring lamp operation, reaches the cathode surface in this metallic formand at the cathode surfaces leads to a drop in the work function of thecathode.

The drop in the work function is associated with a reduction in theoperating temperature of the cathode, which leads to a longer servicelife of the cathode, since less cathode material evaporates at lowertemperatures.

The previously preferred use of ThO₂ as dopant is based on the fact thatthe evaporation of the dopant is relatively slight and therefore doesnot cause extensive disruptive precipitation in the lamp bulb(blackening, deposits). The preferential suitability of ThO₂ correlatesto a high melting point of the oxide (3323 K) and the metal (2028 K).

However, electrode burn-back cannot be avoided even in the case ofthoriated cathodes, and consequently, in the present case of a directcurrent gas discharge lamp, the cathode burn-back imposes limits on theservice life. This is disadvantageous in particular in the case of lampswith short electrode spacings—as are present here—since in this caseeven slight electrode burn-back leads to extensive changes to thelighting properties of the lamp. However, the main drawback of usingThO₂ is its radioactivity, which requires safety precautions to be takenwhen producing the precursor material and the lamp. Depending on theactivity of the product, it is also necessary to comply with regulationsrelating to storage, operation and disposal of the lamps.

It is particularly urgent to solve the environmental problem for lampswith high operating currents of more than 20 A, as are used inmicrolithography or projection technology, since these lamps have aparticularly high activity on account of the electrode size.

Numerous thorium substitutes have therefore been investigated. Examplesof these substitutes are to be found in “Metallurgical Transactions A”,vol. 21A, December 1990, pp. 221-3236. The commercial use of substitutesin lamps for microlithography or cinema projection has not hithertosucceeded, since all substitutes led to pronounced bulb deposits onaccount of the fact that they evaporate more readily than ThO₂.

In microlithography, the productivity of exposure equipment is cruciallydependent on the light quantity provided by the lamp. Bulb deposits andelectrode burn-back reduce the useful light available and lead to a lossof productivity from the very expensive systems, on account ofincreasing exposure times.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a short archigh-pressure discharge lamp in accordance with the preamble of claim 1which makes do without radioactive dopants in the electrode material,ensures low electrode burn-back, is not inferior, or at most onlyslightly inferior, to the proven prior art with regard to electrodeburn-back and, if possible, further reduces the formation of deposits inthe lamp bulb during the lamp service life.

This object is achieved in the case of a short arc high-pressuredischarge lamp having the features of the preamble of claim 1 by virtueof the fact that at least the material of the cathode tip, in additionto the tungsten, contains lanthanum oxide La₂O₃ and at least one furtheroxide selected from the group consisting of HfO₂ and ZrO₂.

Tests carried out on different combinations of dopants had revealed thatthese mixed oxides based on La₂O₃ have favorable results with regard tothe formation of deposits and electrode burn-back. The doping of thecathode tip with La₂O₃ or of the entire cathode should preferably amountto between 1.0 and 3.5% by weight of the cathode material, or preferablybetween 1.5 and 3.0% by weight of the cathode material. It was attemptedto achieve further improvements by adding further oxides or carbides. Inthis context, it was found that the addition of small quantities of ZrO₂and/or HfO₂ makes it possible to achieve a further improvement to theproperties in terms of the emitted evaporation. The molar quantity ofZrO₂ and HfO₂ should in this context advantageously amount to at least2% of the molar quantity of the La₂O₃, but at the same time should notexceed the molar quantity of the La₂O₃, since the favorable influence onthe light flux is always associated with an increased burn-back of thecathode. An excess of La₂O₃ is ensured if the proportion by weight ofHfO₂ amounts to no more than 0.65 times, and/or the proportion by weightof ZrO₂ amounts to no more than 0.38 times, the La₂O₃.

The addition of the second oxide has a significant influence on thelight flux and electrode burn-back while the lamp is operating. Amercury arc lamp with a power of 1.75 kW, an La₂O₃ content in thecathode tip of 2.0% by weight, and a further oxide, revealed thefollowing properties in tests after an operating period of 1500 h: Lightflux based on 0 h = 100% Cathode burn-back Content of second oxide HfO₂in % by weight 0.0% 85% 0.22 mm 0.1% 89% 0.21 mm 0.5% 92% 0.31 mm 1.0%92% 0.43 mm 2.0% 84% 0.55 mm Content of second oxide ZrO₂ in % by weight0.1% 87% 0.25 mm 0.5% 94% 0.29 mm 1.0% 86% 0.52 mm 2.0% 74% 0.83 mm

The following values were observed when using thoriated cathodes (2% byweight of ThO₂): Light flux based Cathode burn- on 0 h = 100% back 94%0.27 mm

The improvement to the light flux in pure xenon arc lamps produced bythe addition of a second oxide in the form of ZrO₂ and/or HfO₂ whenusing La₂O₃-doped cathodes was also detectable. The addition of oxide inthis case too reduces the strong discharge of doping substance, whichleads to rapid formation of deposits on the bulb.

Cathodes made from thorium-free material have a larger arc attachment onaccount of their properties, in particular when using mixed oxides. Theoptimum burn-back of cathodes of this type can be ensured if the plateausize of the cathode is adapted accordingly. If the plateau size were notadapted, either the arc would attach to a plateau edge (if the plateauis too large) or would engage well over the edge of the plateau (plateautoo small). In both cases, without an optimized plateau size, electrodedamage, with an associated increase in burn-back, would be discernible.Since the plateau may be of either planar or curved form, the optimumplateau size can in technical terms best be defined by giving thecurrent density in the cathode at a distance of 0.5 mm behind thecathode tip. Tests carried out on cathodes which were doped with La₂O₃and also with ZrO₂ and/or HfO₂ revealed that the cathode burn-back withthis cathode material can be optimally minimized if the form of thecathodes is such that the current density J in the cathode, i.e. thequotient of lamp current J in A and effective surface area S at adistance of 0.5 mm from the cathode tip toward the rear end of thecathode, is no less than 5 and no greater than 150 A/mm² in the case ofa mercury/noble gas fill and no less than 25 and no greater than 200A/mm² in the case of a pure noble gas fill.

BRIEF DESCRIPTION OF THE DRAWINGS

In the text which follows, the invention is to be explained in moredetail on the basis of an exemplary embodiment. In the drawing:

FIG. 1 shows a mercury short arc high-pressure discharge lamp accordingto the invention in section,

FIG. 2 shows a detailed excerpt from the cathode of the mercury shortarc high-pressure discharge lamp shown in FIG. 1,

FIG. 3 shows a xenon short arc high-pressure discharge lamp according tothe invention, partially in section,

FIG. 4 shows the electrode arrangement of the xenon short archigh-pressure discharge lamp shown in FIG. 3 on an enlarged scale.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows, in section, a mercury short arc high-pressure dischargelamp 1 according to the invention with a power of 1.75 kW. It has a bulb2 made from quartz glass which is shaped elliptically. This is adjoinedon two opposite sides by two ends 3 which are designed as bulb necks 4and each include holding parts 8. The necks have a front conical part 4a, which includes a small supporting roll 5 made from quartz glass asthe main component of the holding part, and a rear cylindrical part 4 b,which forms the fused seal. The front part 4 a has a contraction 6 witha length of 5 mm. This is in each case adjoined by a small supportingroll 5 with a central hole which is conical in shape. Its internaldiameter is 7 mm, its external diameter at the front end is 11 mm, theexternal diameter at the rear end is 15 mm. The wall thickness of thebulb 2 is approximately 4 mm in this region. The axial length of thesmall supporting roll is 17 mm.

A shank 10 of a cathode 7 with an external diameter of 6 mm, whichextends as far as into the discharge volume, where it bears an integralhead part 25, is guided axially in the hole in the first smallsupporting roll. The shank 10 is extended at the rear to beyond thesmall supporting roll 5 and ends at a disk 12, which is adjoined by thefused seal in the form of a cylindrical quartz block 13. This in turn isfollowed by a second disk 14, which in the center holds an externalcurrent feed in the form of a molybdenum rod 15. Four molybdenum foils16 run along the outer surface of the quartz block 13 in a manner whichis known per se and are fused to the wall of the bulb neck in a gastightmanner.

The anode 26, comprising separate head part 18 and shank 19, is held inthe hole in the second small supporting roll 5 in a similar way.

FIG. 2 shows a detail view of the cathode 7 and the holding part 8. Thecathode 7 is composed of a cylindrical shank 10 with a length of 36 mmand a head 25 with a length of 20 mm, the head 25 and the shank havingan external diameter of 6 mm. That end of the head 25 which faces theanode is designed as a tip 11 with a tip angle β of 60° and has aplateau-like end 27 with a diameter of 0.5 mm. The holding partcomprises small supporting roll 5 and a plurality of foils in its hole.

A foil 24 is wound around the shank a number of times (two to fourlayers) in order to mechanically separate the small supporting roll andshank. A pair of narrow foils 23, which lie opposite one another on thewound foil 24, are used for fixing the small supporting roll. For thispurpose, they project beyond the small supporting roll on the dischargeside and are bent over outward. The material of the tip 11 of thecathode 7 includes, in addition to tungsten, a doping of 2.0% by weightof La₂O₃ and 0.5% by weight of ZrO₂.

The mercury short arc high-pressure discharge lamp according to theinvention has a discharge vessel with a volume of 134 cm³, which isfilled with 603 mg of mercury and xenon with a cold-fill pressure of 800mbar.

The operating current of the lamp with an electrode spacing of 4.5 mm is60 A. The current density J in the cathode at a distance of 0.5 mm fromthe plateau tip is 66 A/mm² when the lamp is operating.

FIG. 3 shows a short arc high-pressure discharge lamp 28 according tothe invention with a pure Xe fill. The lamp 28, with a power consumptionof 3 kW, comprises a rotationally symmetrical lamp bulb 29 made fromquartz glass, with a lamp neck 30, 31, likewise made from quartz glass,fitted to each of its two ends. An electrode rod 32 of a cathode 33 isfused in a gastight manner into one neck 30; the inner end of thiselectrode rod bears a cathode head 34. An electrode rod 35 of an anode36 is likewise fused in a gastight manner into the other lamp neck 31and has an anode head 37 secured to its inner end. Cap systems 38, 39for holding and for electrical contact are fitted to the outer ends ofthe lamp necks 30, 31.

As can be seen from FIG. 4, the cathode head 34 is composed of a conicalend section 34 a facing the anode head 37 and an end section 34 b whichfaces the electrode rod 32 and includes a cylindrical subsection and afrustoconical subsection, with a likewise cylindrical section 34 c ofsmaller diameter, referred to as the thermal barrier groove, beinglocated between these two sections 34 a, 34 b. The tip of the conicalend section 34 a, facing the anode head 37, of the cathode head 34, witha cone angle α of 40°, is designed as a hemisphere with a radius R of0.6 mm. The lamp current is in this case 100 A, and the resultingcurrent density at the reference surface 0.5 mm behind the cathode tipis 88 A/mm².

The anode head 37 comprises a cylindrical middle section 37 a with adiameter D of 22 mm and two frustoconical end sections 37 b, 37 c whichface the cathode head 34 and the electrode rod 35, respectively. Thefrustoconical end section 37 c that faces the cathode head 34 has aplateau AP with a diameter of 6 mm. All the sections of the twoelectrodes 33, 36 consist of tungsten. In addition, the conical endsection 34 a of the cathode head 34 includes a doping of 2.0% by weightof La₂O₃ and 0.5% by weight of HfO₂.

The two electrodes 33, 36 are arranged opposite one another on the axisof the lamp bulb 29, in such a way that when the lamp is in the hotstate an electrode spacing or arc length of 3.5 mm results.

1. A short arc high-pressure discharge lamp (1, 28) for direct currentoperation, having a discharge vessel (2, 29) which includes twodiametrically opposite necks (4; 30, 31), into which an anode (26, 36)and a cathode (7, 33), in each case made from tungsten, are fused in agastight manner and which contains a fill comprising at least one noblegas and optionally mercury, characterized in that at least the materialof the cathode tip (11, 34 a), in addition to the tungsten, containslanthanum oxide La₂O₃ and at least one further oxide selected from thegroup consisting of hafnium oxide HfO₂ and zirconium oxide ZrO₂.
 2. Theshort arc high-pressure discharge lamp as claimed in claim 1,characterized in that the cathode material of the entire cathode (7, 34)contains La₂O₃ and at least one further oxide selected from the groupconsisting of HfO₂ and ZrO₂.
 3. The short arc high-pressure dischargelamp as claimed in claim 1, characterized in that the La₂O₃ content ofthe cathode material is from 1.0 to 3.5% by weight.
 4. The short archigh-pressure discharge lamp as claimed in claim 1, characterized inthat the La₂O₃ content of the cathode material is from 1.5 to 3.0% byweight.
 5. The short arc high-pressure discharge lamp as claimed inclaim 1, characterized in that the additional molar quantity ofzirconium oxide ZrO₂ and hafnium oxide HfO₂ does not exceed that of theLa₂O₃ in the cathode material.
 6. The short arc high-pressure dischargelamp as claimed in claim 1, characterized in that the additional molarquantity of zirconium oxide ZrO₂ and hafnium oxide HfO₂ amounts to atleast 2% of the molar quantity of the La₂O₃.
 7. The short archigh-pressure discharge lamp as claimed in claim 1, characterized in theelectrode spacing between anode (26) and cathode (7) in the dischargevessel (2) is less than or equal to 8 mm.
 8. The short arc high-pressuredischarge lamp as claimed in claim 1, characterized in that theelectrode spacing between anode (36) and cathode (33) in the dischargevessel (29) is less than or equal to 15 mm.
 9. The short archigh-pressure discharge lamp as claimed in claim 1, characterized inthat the lamp current when the lamp (1, 28) is operating is greater than20 A.
 10. The short arc high-pressure discharge lamp as claimed in claim1, characterized in that the form of the cathode (7) is such that whenthe lamp is operating the current density J, i.e. the quotient of lampcurrent in A and effective cathode surface area in mm² for an area whichresults from a section through the cathode perpendicular to the lampaxis at a distance of 0.5 mm from the tip of the cathode, satisfies thefollowing equation:≦J≧150 in the case of a mercury/noble gas fill25≦J≧200 in the case of a pure noble gas fill.
 11. The short archigh-pressure discharge lamp as claimed in claim 2, characterized inthat the La₂O₃ content of the cathode material is from 1.0 to 3.5% byweight.
 12. The short arc high-pressure discharge lamp as claimed inclaim 2, characterized in that the La₂O₃ content of the cathode materialis from 1.5 to 3.0% by weight.
 13. The short arc high-pressure dischargelamp as claimed in claim 2, characterized in that the additional molarquantity of zirconium oxide ZrO₂ and hafnium oxide HfO₂ does not exceedthat of the La₂O₃ in the cathode material.
 14. The short archigh-pressure discharge lamp as claimed in claim 2, characterized inthat the additional molar quantity of zirconium oxide ZrO₂ and hafniumoxide HfO₂ amounts to at least 2% of the molar quantity of the La₂O₃.